Fuel cell device

ABSTRACT

A fuel cell device is improved for operating conditions during a partial load operation. The fuel cell device comprises a cell stack formed by electrically connecting fuel cells for generating power by fuel gas and oxygen-containing gas; a fuel gas supply unit for supplying the fuel gas to the fuel cells; and a power adjustment unit for adjusting the amount of current that is supplied to an external load and a controller for controlling the fuel gas supply unit and the power adjustment unit. The controller adjusts, during the partial load operation of the fuel cell device and when the fuel gas supplied to the cell stack is at a low flow rate. The relationship between a fuel utilization rate of the cell stack and the amount of power generated by the cell stack can be nonlinear.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a Divisional of U.S. patent application Ser. No.13/387,499, filed Jan. 27, 2012, now U.S. Pat. No. 10,164,276 B2, whichis a national stage entry of the International Patent Application No.PCT/JP2010/062821, filed on Jul. 29, 2010, which claims priority toJapanese Patent Application No. 2009-176296, filed on Jul. 29, 2009,which are entirely incorporated herein by reference.

FIELD OF THE INVENTION

Embodiments of the present invention generally relate to fuel celldevices, and more particularly relate to a fuel cell device capable ofoperating a partial load operation.

BACKGROUND OF THE INVENTION

In recent years, fuel cell modules including a cell stack having aplurality of fuel cells arranged inside a housing container that arecapable of generating power using fuel gas (gas containing hydrogen) andair (oxygen-containing gas) have been proposed as forms ofnext-generation energy. Various fuel cell devices including the fuelcell modules in an exterior case have been proposed.

Various kinds of fuel cells such as polymer electrolyte fuel cells(PEFC), molten-carbonate fuel cells (MCFC), phosphoric acid fuel cells(PAFC), solid oxide fuel cells (SOFC), and the like are known as suchfuel cells; however, in particular, solid oxide fuel cells may easilyfollow partial loads for household use.

The partial load operation is generally known to decrease the amount offuel gas supplied to the cell stack (fuel cells).

However, during the partial load operation of the fuel cell device, arate of fuel utilization and amount of current (electric powergeneration) of the cell stack fluctuate (declined compared to during therated operation) according to an external load.

Therefore, there is a need for a more efficient fuel cell device withimproved operating conditions during the partial load operation.

SUMMARY OF THE INVENTION

A fuel cell device comprising a cell stack and methods are disclosed.The cell stack includes a plurality of fuel cells that is operable togenerate power as a result of a reaction of a fuel gas and anoxygen-containing gas. A relationship between a rate of the fuelutilization of the cell stack and the amount of electrical currentgenerated by the cell stack is controlled to allow the fuel cell deviceto have efficient partial load operation.

In one embodiment, a fuel cell device according to the present inventionincludes a cell stack, a fuel gas supply unit, a power conditioning unitand a controller. The cell stack comprises a plurality of fuel cellselectrically coupled to generate power with a fuel gas and anoxygen-containing gas. The fuel gas supply unit supplies the fuel gas tothe fuel cells. The power conditioning unit controls the amount ofelectrical current that is generated by the fuel cells and that issupplied to the external load. The controller controls the fuel gassupply unit and the power conditioning unit such that the relationshipbetween a rate of the fuel utilization of the cell stack and the amountof electrical current generated by the cell stack is nonlinear if thefuel gas supplied to the cell stack is the minimum flow or morenecessary for generating power during the partial load operation of thefuel cell device.

In another embodiment, a fuel cell device comprises: a cell stackcomprising a plurality of fuel cells electrically coupled, and operableto generate electrical current from a reaction of a fuel gas and anoxygen-containing gas; and a controller operable to maintain arelationship between a rate of fuel utilization of the cell stack and anamount of the electrical current non-linear during a partial loadoperation of the fuel cell device if the fuel gas flows into the cellstack at least a minimum flow necessary for generating power.

In a further embodiment, a method of operating a fuel cell devicecomprising fuel cells in a partial load operation is described. In themethod, a fuel gas is supplied to the fuel cells at least a minimum flownecessary for generating power; an oxygen-containing gas is supplied tothe fuel cells; an electrical current is generated at the fuel cells asa result of a reaction of the fuel gas and the oxygen-containing gas;and the relationship between a rate of fuel utilization of the fuelcells and the electrical current non-linear is maintained during apartial load operation.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the present disclosure are hereinafter described inconjunction with the following figures, wherein like numerals denotelike elements. The figures are provided for illustration and depictexemplary embodiments of the present disclosure. The figures areprovided to facilitate understanding of the present disclosure withoutlimiting the breadth, scope, scale, or applicability of the presentdisclosure.

FIG. 1 is an illustration of an exemplary schematic diagram of a fuelcell system including a fuel cell device according to an embodiment ofthe present invention.

FIG. 2 is a graph illustrating a relationship between a rate of fuelutilization of a cell stack and an amount of current generated by thecell stack according to the request of the external load in a fuel celldevice according to an embodiment of a present invention.

FIG. 3 is a graph illustrating a relationship between a rate of fuelutilization of a cell stack and an amount of current generated by thecell stack according to the request of the external load in a fuel celldevice according to an embodiment of a present invention.

FIG. 4 is a graph illustrating a relationship between a rate of fuelutilization of a cell stack and an amount of current generated by thecell stack according to the request of the external load in a fuel celldevice according to an embodiment of a present invention.

DETAILED DESCRIPTION OF THE DRAWINGS

The following description is presented to enable a person of ordinaryskill in the art to make and use the embodiments of the disclosure. Thefollowing detailed description is exemplary in nature and is notintended to limit the disclosure or the application and uses of theembodiments of the disclosure. Descriptions of specific devices,techniques, and applications are provided only as examples.Modifications to the examples described herein will be readily apparentto those of ordinary skill in the art, and the general principlesdefined herein may be applied to other examples and applications withoutdeparting from the spirit and scope of the disclosure. The presentdisclosure should be accorded scope consistent with the claims, and notlimited to the examples described and shown herein.

FIG. 1 is an illustration of an exemplary schematic diagram of a fuelcell system including a fuel cell device according to an embodiment ofthe present invention.

An embodiment of a fuel cell system illustrated in FIG. 1 may include apower generating unit to generate power, a hot water storage unit tostore hot water obtained as a result of heat exchange, and a circulationpipeline to circulate water among these units.

The power generating unit illustrated in FIG. 1 may include a cell stack1 including a plurality of fuel cells arranged and electrically coupled(not illustrated), a raw fuel supply unit 2 to supply raw fuel such as anatural gas and the like, an oxygen-containing gas supply unit 3 tosupply oxygen-containing gas to the fuel cells constituting the cellstack 1, and a reformer 4 that carries out a steam-reforming reactionfrom the raw fuel as well as steam. The reformer 4 may include avaporizing unit (not illustrated) to vaporize pure water supplied from awater pump 5 mentioned later and to mix the raw fuel supplied from theraw fuel supply unit 2 with the steam, and reforming within, and furthermay include a reforming unit (not illustrated) including catalysttherein to generate a fuel gas (hydrogen-containing gas) by reacting themixed raw fuel with the steam. Thereby, the electric power is generatedin the fuel cells (cell stack 1) by a reaction of the fuel gas generatedat the reformer 4 and the oxygen-containing gas supplied from theoxygen-containing gas supplying unit 3. The fuel gas supply unit mayinclude the raw fuel supply unit 2.

The cell stack 1 and the reformer 4 are located inside the housingcontainer and are configuring a fuel cell module (hereinafter may beabbreviated as module). In FIG. 1, some units included in the fuel cellmodule are surrounded by dotted and dashed lines and M indicates themodule. However, the reformer 4 may be provided outside of the housingcontainer.

Here, the module M is described. Known fuel cell modules may be used forthe module M. For example, in one embodiment, columnar-shaped fuel cellsincluding a gas passage in which gas circulates inside are arrangedinside the housing container in an up-right state, configuring the cellstack 1 by electrically connecting neighboring fuel cells in series viapower-collecting members. Furthermore, the module M includes a cellstacking device and the reformer 4. In the cell stacking device, eachbottom end of fuel cells is fixed to a fuel gas chamber with insulatingbonding materials such as glass sealing material, and the like. Thereformer 4 is located on or above the fuel cells to supply the fuel gasto fuel cells.

Various fuel cells are known as fuel cells constituting the cell stack1; however, in conducting a partial load operation (load followoperation), solid oxide fuel cells may be selected. Auxiliariesnecessary for the movement of fuel cells may be downsized by selectingsolid oxide fuel cells as the fuel cells composing the cell stack 1,allowing the fuel cell device to be downsized.

The fuel cells may have various shapes such as flat types, cylindricaltypes, and the like. However, upon efficiently generating electric powerat fuel cells, a hollow flat fuel cell may be selected. Fuelelectrode-supporting type of hollow flat fuel cells with a fuelelectrode layer formed to the inside and an oxygen electrode layerformed to the outside may be used as such hollow flat fuel cells.

The power generating unit, as illustrated in FIG. 1, may include a heatexchanger 6, a condensed water purifier 7 and a condensed water feedingtube 15. The heat exchanger 6 exchanges heat with an exhaust gas(exhaust heat) produced from the generation of electric power at fuelcells constituting the cell stack 1 and water flowing through acirculation pipeline 13. The condensed water purifier 7 purifies(preferably generates pure water) the condensed water generated fromheat exchange. The condensed water feeding tube 15 supplies thecondensed water generated at the heat exchanger 6 to the condensed waterpurifier 7 are formed. The condensed water processed at the condensedwater purifier 7 is stored in a water tank 8 connected by atank-connecting pipe 16 and subsequently supplied to the reformer 4(vaporizing unit, not illustrated) by the water pump 5. The water tank 8may be omitted by having the condensed water purifier 7 function as thewater tank.

The power generating unit, as illustrated in FIG. 1, may include a powerconditioning unit 9, an outlet water temperature sensor 11, a controller10, and a circulating pump 12. The conditioning unit 9 convertsdirect-current power generated at the fuel cells into analternating-current power and controls the amount of convertedalternating-current power to the external load. The outlet watertemperature sensor 11 is located at an outlet of the heat exchanger 6and measures the temperature of water flowing through the outlet of theheat exchanger 6 (circulating water flow). The circulating pump 12circulates water within a circulation pipeline 13. Then, the fuel celldevice may include the cell stack 1, the controller 10, a fuel gassupplying unit to supply the fuel gas to the fuel cells and a powerconditioning unit to control the amount of electrical current generatedat the fuel cells and supplied to the external load.

In FIG. 1, the connection of a power conditioning unit 9 and theexternal load is omitted, and a power conditioner may be exhibited as anexample of the power conditioning unit 9. Consequently, installation,carrying, and the like, may be simplified by housing each of thesedevices that constitutes the power generating unit inside the exteriorcase. The hot water storage unit may include a hot water storage tank 14to store hot water after heat exchange.

Pollution-abatement equipment for exhaust gas (not illustrated), whichprocesses the exhaust gas with the operation of the cell stack 1, islocated between the cell stack 1 and the heat exchanger 6. In thepollution abatement equipment for exhaust gas, an exhaust gas processingunit is inside the housing container and a known combustion catalyst maybe used as the exhaust gas processing unit.

Meanwhile, if an amount of condensed water supplied to the condensedwater purifier 7 is small and/or if the condensed water processed at thecondensed water processing unit is of low purity, water supplied fromthe outside (tap water or the like) may be purified and supplied to thereformer 4. In FIG. 1, external water purification equipment X includingone or more water processing units to purify water supplied from theoutside are equipped.

Here, the external water purification equipment X supplies the waterfrom the outside to the reformer 4. The external water purificationequipment X may include an ion-exchange resin unit 2, and may furtherinclude an activated charcoal filtering device 19 and/or a reverseosmosis unit 20. Then, the pure water generated at the water processingunit is stored in the water tank 8. The fuel cell device (powergenerating unit) illustrated in FIG. 1 may further include a feed valve18 to adjust the amount of water supplied from the outside.

In FIG. 1, the external water purification equipment X is surrounded bya dashed line (indicated as X).

The external water purification equipment X may be omitted.Specifically, if the external water purification equipment X may beomitted if the water (pure water) necessary for the steam-reformingreaction at the reformer 4 is maintained by the condensed water alonegenerated from heat exchange between the exhaust gas (exhaust heat)produced by the power generation at the fuel cells and the water from acirculation pipeline 13.

Here, a method of operating the fuel cell device (power generating unit)illustrated in FIG. 1 is described. When carrying out steam reformingreaction in order to produce the fuel gas used for power generation atfuel cells, the condensed water produced by heat exchange between theexhaust gas produced by the operation of the cell stack 1 (fuel cells)at the heat exchanger 6 and the water flowing inside the circulationpipeline 13, is used as the pure water used in the reformer 4. The waterflows inside the circulation pipeline 13 to increase water temperaturedue to heat exchange with the exhaust gas (that is to say, hot water),and then is stored in the hot water storage tank 14.

The condensed water produced at the heat exchanger 6 flows inside acondensed water supplying pipe 15 and is supplied to the condensed waterpurifier 7. The condensed water (pure water) processed at the condensedwater purifier 7 (ion-exchange resin, and the like) is supplied to thewater tank 8 through a tank connecting pipe 16. The water stored in thewater tank 8 is supplied to the reformer 4 by the water pump 5. Then,steam reforming of the raw fuel supplied from the raw fuel supply unit 2is carried out in the reformer 4 with the water. Consequently, theproduced fuel gas is supplied to the fuel cells (cell stack 1).

Electric power is generated in the fuel cells (cell stack 1) with usingthe fuel gas supplied from the reformer 4 and the oxygen-containing gassupplied from the oxygen-containing gas supplying unit 3, and anelectrical current generated at the fuel cells (cell stack 1) issupplied to the external load via an adjuster 9. Due to the methodsmentioned above, autonomous water operation may be carried out byefficiently making use of the condensed water.

Outside water supplied from the outside (tap water and the like) may beused for the steam reforming. Specifically, the outside water may beused if little condensed water is produced or if the condensed waterprocessed at the condensed water purifier 7 is of low purity.

In such cases, first, the feed valve 18 (for example, solenoid valve,air-driving valve, or the like) opens, and the outside water is suppliedto the activated charcoal filter 19 via a water pipe 17. The waterprocessed at the activated charcoal filter 19 is subsequently suppliedto a reverse osmosis membrane 20. The water processed at the reverseosmosis membrane 20 is subsequently supplied to the ion-exchange resinunit 21. Then, the water purified at the ion-exchange resin unit 21 isstored in the water tank 8. The purified water (pure water) stored inthe water tank 8 is used for generating electric power at the fuel cells(cell stack 1) by the method mentioned above.

In fuel cell devices having a configuration such as those mentionedabove, the controller 10 controls the operation of the raw fuel supplyunit 2 and the oxygen-containing gas supplying unit 3 during the ratedoperation, and supplies the amount of fuel gas and oxygen-containing gasnecessary for rated operation to the fuel cells (cell stack 1). Thereby,a rated power is generated in the fuel cells (cell stack 1) and directcurrent flows in the fuel cells (cell stack 1). The electric powergenerated by the electrical generation at the fuel cells (cell stack 1)is supplied to the external load after being converted toalternating-current power at the adjuster 9. That is to say, thecontroller 10 controls each unit during the rated operation such thatthe relationship between a rate of fuel utilization (Uf) of the cellstack 1 and the amount of current (I) generated by the cell stack 1becomes a constant rate in compliance with the demands from the externalload.

When using the fuel cell device for household purposes, the requiredpower of the external load is prone to fluctuate. The required powerbecomes higher particularly in the early morning and evening onwards,causing the electrical current flowing in the cell stack 1 to be higher;whereas, in the day time or at midnight, the required power becomeslower, causing the electrical current flowing in the cell stack 1 to besmaller.

Having the fuel cell device carry out the rated operation in a time zonewith low required power may cause a reverse power flow of theelectricity from the fuel cell device in the system power connected tothe fuel cell device. Therefore, particularly in the operation of thefuel cell device for household purposes, the partial load operation(load follow operation) corresponding to the required power of theexternal load may be carried out.

During such a partial load operation, the controller 10 controls theoperations of the raw fuel supply unit 2 and the oxygen-containing gassupplying unit 3, and supplies the amount of fuel gas andoxygen-containing gas necessary for obtaining an amount of currentcorresponding to the required power of the external load to the fuelcells (cell stack 1). The direct-current power resulting from thegeneration of electric power by the fuel cells (cell stack 1) isconverted to alternating-current power at the power conditioning unit 9and subsequently supplied to the external load.

That is to say, the rate of fuel utilization (Uf) and the amount ofcurrent (I) of the cell stack 1 fluctuates in correspondence with therequired load during partial load operation. Specifically, the rate andthe amount decline more compared to during the rated operation.

Therefore, unless the relationship between the rate of fuel utilization(Uf) and the amount of current (I) of the cell stack 1 is controlled inproper balance during the partial load operation, a danger of the powergeneration efficiency of the fuel cell device may decline and/or theload-following characteristic may decline. FIG. 2 to FIG. 4 are graphsillustrating the relationship between rates of fuel utilization of thecell stack 1 in the fuel cell device and the amounts of currentgenerated by the cell stack 1 according to the request of the externalload.

Since it is necessary to maintain the temperature of the fuel cellsabove a predetermined temperature in maintaining the operation of thefuel cell device, even if the required power of the external load islow, it may be necessary to supply the fuel gas at a predeterminedamount or more to the fuel cells (cell stack 1). Hereinafter, thepredetermined amount of fuel gas is referred to as the minimum flow.

In the fuel cell device according to the present embodiment, if the fuelgas supplied to the cell stack 1 during the partial load operation isthe minimum flow or more, the controller 10 controls the raw fuel supplyunit 2 and the power conditioning unit 9 such that the relationshipbetween the rate of fuel utilization (Uf) of the cell stack 1 and theamount of current (I) generated by the cell stack 1 in response to therequest of the external load becomes non-linear. In addition, thecontroller preferably controls the oxygen-containing gas supplying unit3 together, and the same applies hereinafter.

That is to say, if the raw fuel supply unit 2 and the power conditioningunit 9 are controlled such that the relationship between the rate offuel utilization (Uf) and the amount of current (I) of the cell stack 1becomes linear during partial load operation, as mentioned later, itbecomes difficult to carry out operations such as the operation toimprove the load-following characteristic, the operation to controlaccidental fires when burning excess fuel gas at one end of the fuelcells.

In contrast, if the fuel gas supplied to the cell stack 1 during partialload operation is at least the minimum flow, a control of the raw fuelsupply unit 2 and the adjuster 9 by the controller 10 in which therelationship between the a rate of fuel utilization (Uf) and the amountof current (I) of the cell stack 1 becomes non-linear can carry outoperations such as the operation to improve the load-followingcharacteristic, the operation to control accidental fires when burningexcess fuel gas at one end of the fuel cells during the partial loadoperation, thereby making it possible to carry out efficient partialload operation.

If a maximum rate of fuel utilization (Uf) of the cell stack 1 duringthe partial load operation is the same as the rate of fuel utilization(Uf) during the rated operation of the fuel cell device, oxidation ofthe fuel cells may be reduced, thereby reducing damage to the fuelcells. Consequently, a fuel cell device with increased credibility maybe obtained.

Then, as illustrated in FIG. 2, if fuel gas more than the minimum flowof the fuel gas supplied to the cell stack 1 is supplied to the cellstack 1 during partial load operation, when the controller 10 controlseach of the fuel gas supply unit 2 and the power conditioning unit 9such that the amount of increase in the rate of fuel utilization (Uf) ofthe cell stack 1 is reduced along with the increase in the amount ofcurrent (I) generated by the cell stack 1, the fuel cells may bemaintained at a high temperature even if the amount of current (I) ofthe cell stack 1 is low, thereby improving the load-followingcharacteristic of the fuel cell device.

Meanwhile, in the fuel cell device with a configuration in which theexcess fuel gas, which was not used in generating electric power by thefuel cells, can be burned at one end of the fuel cells, the excess fuelgas may accidentally combust due to the declined amount of fuel gassupplied to the fuel cells (cell stack 1) during partial load operation.

Therefore, as illustrated in FIG. 3, if the fuel gas supplied to thecell stack 1 during partial load operation is at least the minimum flow,when the raw fuel supply unit 2 and the power conditioning unit 9 arecontrolled by the controller 10 such that the amount of increase of therate of fuel utilization (Uf) increases along with the increase ofamount of current (I) generated by the cell stack 1, the excess fuel gasincreases when the amount of current (I) of the cell stack 1 is low.That is to say, accidental combustion of excess fuel gas may bedecreased due to increase of the excess fuel gas.

In FIG. 2 and FIG. 3, the relationship between the rate of fuelutilization (Uf) of the cell stack 1 and amount of current (I) is aquadratic curve during the partial load operation when fuel gas of morethan the minimum flow of fuel gas supplied to the cell stack 1 issupplied to the cell stack 1, but the relationship is not limited to therelationship expressed by the quadratic curve. It may be setappropriately according to the amount of fuel cells configuring the fuelcell device, the size of the module M, or the like, and for example, therelationship may be expressed by a cubic curve, or the like.

For example, as illustrated in FIG. 4, if the fuel gas supplied to thecell stack 1 is at least the minimum flow, the controller 10 may controlthe raw fuel supply unit 2 and the power conditioning unit 9 such thatan amount of increase in the rate of fuel utilization (Uf) increasesalong with the increase in amount of current (I) generated by the cellstack 1, and subsequently, the amount of increase of the rate of fuelutilization (Uf) decreases along with the increase in amount of current(I). In such a case, the relationship between the rate of fuelutilization (Uf) and the amount of current (I) of the cell stack 1during the partial load operation is expressed by the cubic curve.

Thereby, excess fuel gas increases in the region with low amount ofcurrent (I) of the cell stack 1 and accidental combustion of the excessfuel gas in the fuel cell device may be prevented while improving theload-following characteristic.

The relationship between the rate of fuel utilization (Uf) and theamount of current (I) of the cell stack 1 is linear until the minimumflow of fuel gas supplied to the cell stack 1 is reached.

The present invention was described in detail but the present inventionis not limited to the embodiment mentioned above, and various changes,revisions, and the like, are possible within the range that does notdeviate from the purpose of the present invention.

For example, if fuel gas greater than the minimum flow is supplied tothe cell stack 1 during partial load operation, the controller 10 maycontrol the raw fuel supply unit 2 and the power conditioning unit 9such that the amount of increase in the rate of fuel utilization (Uf)decreases along with the increase in the amount of current (I) generatedby the cell stack 1, and subsequently, the amount of increase in therate of fuel utilization (Uf) increases along with the increase in theamount of current (I) generated by the cell stack 1.

What is claimed is:
 1. A fuel cell device comprising: a cell stackcomprising a plurality of fuel cells electrically coupled to generatepower with a fuel gas and an oxygen-containing gas; a fuel gas supplyunit to supply the fuel gas to the plurality of fuel cells; a powerconditioning unit that controls the amount of electrical current that isgenerated by the plurality of fuel cells and that is supplied to theexternal load; and a controller configured to control each of the fuelgas supply unit and the power conditioning unit and is furtherconfigured to: linearly increase a rate of fuel utilization of theplurality of fuel cells with increasing the electric current when thefuel gas flow is below a minimum fuel gas flow necessary to maintain atemperature of the plurality of fuel cells above a predeterminedtemperature; non-linearly increase the rate of fuel utilization from apoint of minimum fuel gas flow with increasing the electric currentduring a partial load operation; and maintaining during a ratedoperation the rate of fuel utilization constant with increasing theelectric current.
 2. The fuel cell device according to claim 1, whereinthe controller is further configured to: decrease during the partialload operation a gradient of the rate of fuel utilization over theelectric current with increasing the electrical current.
 3. The fuelcell device according to claim 1, wherein the controller is furtherconfigured to: increase during the partial load operation a gradient ofthe rate of fuel utilization over the electric current with increasingthe electrical current.
 4. A fuel cell device comprising: a cell stackcomprising a plurality of fuel cells electrically coupled to generatepower with a fuel gas and an oxygen-containing gas; a fuel gas supplyunit to supply the fuel gas to the plurality of fuel cells; and acontroller configured to control the amount of electrical current thatis generated by the plurality of fuel cells and that is supplied to theexternal load, wherein the controller is further configured to: linearlyincrease a rate of fuel utilization (Uf (%)) of the plurality of fuelcells with increasing the electric current when the fuel gas flow isbelow a minimum fuel gas flow necessary to maintain a temperature of theplurality of fuel cells above a predetermined temperature; non-linearlyincrease the rate of fuel utilization (Uf (%)) from a point of minimumfuel gas flow with increasing the electric current during a partial loadoperation; and maintain the rate of fuel utilization (Uf (%)) constantwith increasing the electric current during a rated operation.
 5. Thefuel cell device according to claim 4, wherein the controller is furtherconfigured to: decrease during the partial load operation a gradient ofthe rate of fuel utilization over the electric current with increasingthe electrical current.
 6. The fuel cell device according to claim 4,wherein the controller is further configured to: increase during thepartial load operation a gradient of the rate of fuel utilization overthe electric current with increasing the electrical current.
 7. A fuelcell device comprising: a cell stack comprising a plurality of fuelcells electrically coupled to generate an electric current (I) from areaction of a fuel gas and an oxygen-containing gas in one of at least afirst range defining a partial load operation and a second rangedefining a rated operation; a fuel gas supply unit to supply the fuelgas to the plurality of fuel cells at a fuel gas flow rate from a rawfuel supply unit; and a controller configured to: control the reactionof the fuel gas and the oxygen-containing gas and control the fuel gasflow rate to: linearly vary a rate of fuel utilization (Uf (%)) of theplurality of fuel cells in direct proportion to the electric currentwhen the fuel gas flow is below a minimum fuel gas flow, wherein theminimum fuel gas flow is defined as a minimum fuel gas flow sufficientto maintain a temperature of the plurality of fuel cells above apredetermined temperature; non-linearly vary the rate of fuelutilization (Uf (%)) as a function of the current when the fuel gas flowis above the minimum fuel gas flow and the current is in the firstrange; and maintain the rate of fuel utilization (Uf (%)) constant whenthe current is in the second range.
 8. The fuel cell device according toclaim 7, wherein the current produced during the rated operation ishigher than the current produced during the partial load operation. 9.The fuel cell device according to claim 7, wherein the controller isfurther configured to: decrease, during the partial load operation, agradient of the rate of fuel utilization over the electric current withincreasing the electrical current within the first range.
 10. The fuelcell device according to claim 7, wherein the controller is furtherconfigured to: increase, during the partial load operation, a gradientof the rate of fuel utilization over the electric current withincreasing the electrical current within the first range.